Gallium nitride (GaN) is a potential material to replace Silicon (Si) in high power applications. GaN has a high breakdown voltage, excellent transport properties, fast switching speed and good thermal stability. Gallium nitride is also more cost effective than silicon carbide. Another advantage is that the hetero-structure formed from aluminum gallium nitride (AlGaN) and gallium nitride gives rise to a two-dimensional channel of high-mobility electrons, which enable the GaN devices to achieve a lower on-resistance at the same reverse bias than silicon and silicon carbide.
GaN may provide a technology platform for a wide variety of different semiconductor devices including, for example, diodes and transistors. Diodes are used in a wide range of electronic circuits. Diodes used in circuits for high voltage switching applications ideally require the following characteristics. When biased in the reverse direction (i.e., the cathode is at a higher voltage than the anode), the diode should be able to support a large voltage while allowing as little current as possible to pass through. The amount of voltage that must be supported depends on the application; for example, many high power switching applications require diodes that can support a reverse bias of at least 600 V without passing a substantial amount of current. Finally, the amount of charge stored in the diode when it is reverse biased should be as small as possible to reduce transient currents in the circuit when the voltage across the diode changes, thereby reducing switching losses.
FIG. 1 shows an example of a conventional GaN-based diode. The diode 100 includes a substrate 110, a GaN buffer layer 120, a GaN epitaxial (“epi”) layer 130 and an aluminum gallium nitride barrier layer 135. A first metal layer forms a Schottky contact 140 to the aluminum gallium nitride barrier layer 135 and a second metal layer forms an Ohmic contact 150 to the aluminum gallium nitride barrier layer 135. The Schottky contact 140 serves as the device anode and the Ohmic contact 150 serves as the device cathode. A passivation layer 160 is located between the Schottky contact 140 and the Ohmic contact 150.
A quantum well is formed at the hetero junction interface between the AlGaN layer, which has a large band gap, and the GaN layer, which has a narrower band gap. As a result, electrons are trapped in the quantum well. The trapped electrons are represented by a two-dimensional (2DEG) electron gas 170 in the GaN epi layer and as a consequence electrons flow along the channel between the anode and cathode. Thus, because its operation is based on a two-dimensional electron gas, the charge carriers in the channel establish a current in the lateral direction.
One problem with the diode shown in FIG. 1 is that its on-resistance is relatively large due to the added total contact resistance since the forward current of the 2DEG electron gas has to channel through the AlGaN barrier layer 135 to reach the cathode 150, where the Ohmic contact resistance to GaN-based materials is generally significantly higher compared to silicon. Additionally, because the anode and cathode are located on the same face of the device, the die area that is required is also relatively large. Moreover, the diode's thermal performance is relatively poor because thermal dissipation is limited to only one side of the die.